This invention relates to sails, and in particular to a double panel sail.
Conventional sails, like those still in use today, typically comprise a fabric panel which is constructed with a particular draft--or depth and curvature--so that when properly trimmed it assumes a curved shape with a convex leeward side and a concave windward side. The velocity of the air on the convex leeward side of the sail is generally greater than the velocity of the air on the concave windward side of the sail. Since pressure generally decreases as the velocity increases, the pressure is relatively lower on the leeward side of the sail than on the windward side of the sail. In addition, the wind acts on the windward side of the sail to produce a force on the windward side of the sail. The resulting strong relative negative pressure (or suction) on the leeward side of the sail and the moderate positive force on the windward side of the sail produces a thrust on the mast that propels the sail boat.
The total driving force on the sail boat is a function of the total sail area. Thus sails are sized and shaped to maximize sail area, and the boat and rigging are designed to permit the use of various types of sails such as stay sails, jibs, and other filler sails. The height of the sails must be limited to prevent excessive heeling of the boat and to keep the mast size within practical limits. Similarly the breadth of the sails must be limited to keep the boom and the rigging within practical limits. The use of stay sails is also limited, particularly in smaller boats, due to the limitations of the rigging and the ability of the crew to handle the additional sails. Space is another limitation on the use of stay sails since it is common practice to have several suits of sails for the various wind conditions that may be encountered. The number of these stay sails is compounded by this practice, presenting a storage and handling problem, particularly on a small boat.
The draft of a sail is the source of sail power going windward. The draft is designed taking into account the stability of the hull, the intended use of the boat, and the anticipated wind and sea conditions. The design of the draft also takes into account maximizing boat speed while minimizing the pointing angle (the angle of the boat with respect to the apparent wind which determines how efficiently the boat can "beat" or sail into the wind). The shape of the curvature, including the position of the point of maximum curvature, and the depth of curvature (the camber ratio), are designed for the particular boat. Maintaining this designed draft for the sail is important to efficient sailing.
It is difficult to construct the designed sail shape because the sail materials are lightweight and flexible, and as a consequence tend to stretch. Stiffer, more stretch-resistant materials tend to have less tear resistance and are also more difficult to handle. A great deal of attention has been devoted to the mechanics of sail construction to minimize sail distortion. Careful selection of the material and the use of composites and multiply materials is one way of combating stretch. It is also known that cloth stretches differently in different directions, and thus selection of the weave and careful orientation of the material is another way of controlling sail stretch. Miter cut, radial cut, spider web cut, and star cut sail constructions are examples of attempts to minimize sail distortion by optimizing the orientation of the cloth in the sail. Finally there are various finishing processes that can reduce in-service distortion of the sail.